1. Field of the Invention
This invention relates to transparent materials that are capable of absorbing liquids, and, more particularly, to materials that can be used as ink-receptive layers for transparent imageable materials.
2. Discussion of the Art
Transparent materials that are capable of absorbing significant quantities of liquid, while maintaining some degree of durability and transparency, are useful in contact lenses, priming layers for coatings coated out of aqueous solutions, fog-resistant coatings, and transparent imageable materials for use with mechanized ink depositing devices, such as pen plotters and ink-jet printers. Transparent imageable materials are used as overlays in technical drawings and as transparencies for overhead projection. It is desirable that the surface of liquid absorbent materials for use in transparent graphical applications be tack free to the touch even after absorption of significant quantities of ink.
During normal use of pen plotters and ink-jet printers, the inks used in such machines are exposed to open air for long periods of time prior to imaging. After such exposure to air, the ink must still function in an acceptable manner, without loss of solvent. To meet this requirement, ink formulations typically utilize solvents of very low volatility, such as water, ethylene glycol, propylene glycol, and so on. Inks that contain water or water-miscible solvents are commonly referred to as aqueous inks, and the solvents for these inks are commonly referred to as aqueous liquids. Materials that are receptive to such aqueous liquids will hereinafter be referred to as hydrophilic compositions.
Because of the low volatility of aqueous liquids, drying of an image by means of evaporation is very limited. In the case of imaging onto a paper sheet which has a fibrous nature, a significant amount of the liquid diffuses into the sheet, and the surface appears dry to the touch within a very short time. In the case of imaging onto polymeric film, some means of absorbing aqueous liquids is needed if satisfactory drying of the image is to occur.
Compositions useful as transparent liquid absorbent materials have been formed by blending a liquid-insoluble polymeric material with a liquid-soluble polymeric material. The liquid-insoluble material is presumed to form a matrix, within which the liquid soluble material resides. Examples of such blends are the transparent water-absorbent polymeric materials disclosed in U.S. Pat. Nos. 4,300,820, 4,369,229, and in European Patent Application No. 0 233 703.
A problem that frequently arises in the formulation of polymer blends is the incompatibility of the polymers being blended. When attempts are made to blend polymers that are incompatible, phase separation occurs, resulting in haze, lack of transparency, and other forms of inhomogeneity.
Compatibility between two or more polymers in a blend can often be improved by incorporating into the liquid-insoluble matrix-forming polymer chains monomeric units that exhibit some affinity for the liquid-soluble polymer. Polymeric materials having even a small amount of acid functionality are more likely to exhibit compatibility with polyvinyl lactams. Generally, the compatibility of polymers being blended is improved if the polymers are capable of hydrogen bonding to one another.
A second form of incompatibility noted in using blends of liquid-absorbent polymers is the incompatibility of the matrix forming insoluble polymer with the liquid being absorbed. For example, if the liquid being absorbed is water, and if the water-insoluble polymers are hydrophobic, some inhibition of water absorption ability can be expected. One method of overcoming this difficulty is to utilize hydrophilic matrix polymers that are water-insoluble at the temperatures at which they are to be used, though they may be water-soluble at a different temperature. In U.S. Pat. No. 4,503,111, ink-receptive coatings comprising either polyvinyl alcohol or gelatin blended with polyvinyl pyrrolidone are disclosed. Both polyvinyl alcohol and gelatin, being water-insoluble at room temperature, are able to act as matrix-forming polymers for these coatings, and the coatings are quite receptive to aqueous inks. However, the coatings do exhibit a tendency to become tacky, either because of imaging, or because of high humidity.
It therefore becomes clear that while blends of soluble and insoluble polymers may be useful as liquid absorbent compositions, they suffer major limitations in liquid absorption ability and in durability.